Oh I'm glad you liked that path so much! I'm really proud of the possessed sequence myself. I experimented and I thinked it worked out.
spoiler:
To get the hidden ending you have to think about tails at the start of the sequence, and cohen at the end. Not taking any damage in between, because that will kill you on the second thought. It's the option to rebuke your Matron, finally realizing why she has gifted you a tail, and drawing a negative conclusion about the relationship between you.
Re: pregnancy
That'd be an interesting element to be sure and it would perhaps make an interesting epilogue thing, although I don't think that'd fit well with the story I'm telling.
The purely pragmatic reason it didn't come up again is I didn't think about it.
The in-story reason is that even when you don't blow up at her, you immediately dip into steaming hot water and do your best to remove the offending substance. I'm not an expert at such things but that seems a believable anti-pregnancy measure. The thinking about how she can impregnate people with her mind was meant to be a joke.
The reason why it's given so much narrative weight is that it's meant to both poke fun at the story and game this is, and to illuminate in the most elemental sense the relationship between the player character and your matron.
The poking fun is that well, this is a porn game about adventurers doing adventuring. Heroic fantasies rarely focus on how unprotected sex can really screw over the woman, and suddenly being the woman in that equation is meant to point out that lack of thought.
The relationship dynamic is that, as the narration points out, that's not really done among equals, or at least among peasants because you don't screw over your neighbor, especially without asking. If you wished to be charitable, she assumes that you are in a relationship with her and that her trying to impregnate you is just part of that. If you want to be uncharitable, it'll be easier to be in a relationship with you if you are pregnant with her child. Either way, she is assuming that she's the important person in the relationship, and that you will just go with it. There is nothing that she can do, that you won't forgive, as exemplified by her callousness here. She is perhaps thinking with her dick.